Fuel injected engines have gained favor with automotive manufacturers and the general public due to their superior performance, lower level of undesirable emissions, and increased fuel efficiency relative to conventional carbureted engines. Furthermore, fuel injection provides the ability to more accurately control a variety of engine operating parameters via an on-board electronic control unit. In fuel injected engines, fuel is typically supplied to the injectors through one or more rigid conduits, or fuel rails. Fuel is supplied to a fuel rail through a fuel inlet which is integral with or in communication with the fuel rail. A fuel rail is typically adapted to receive fuel injectors at locations, or ports, spaced at intervals along its length. The locations, or spacing, of the ports and fuel injectors are chosen so as to be in alignment with the corresponding intake ports of a particular internal combustion engine with which the fuel rail is intended to be used.
Many fuel injection systems, such as, for example, those used on marine engines, incorporate a vapor separator which receives fuel pumped from the fuel tank at low pressure from an engine driven mechanical lift pump. A high pressure electric fuel pump is provided, either integral with or separate from the vapor separator, to increase the pressure of the fuel to the level required by the fuel injection system prior to entering the fuel rail. The pressurized fuel is delivered to the fuel injectors by the fuel rail. Typically, the high pressure fuel lines are restricted to the area adjacent to the engine. In an outboard motor, any fuel leaking from the high pressure fuel lines adjacent to the engine drains into and is diluted by the body of water in which the boat and engine are located, rather than accumulating in the hull of the boat and creating a fire hazard. Leaks of fuel from inboard motors having this type of fuel system architecture are confined to the engine compartment.
The high pressure fuel pump and the engine itself each dissipate power in the form of heat. The heat can bring the low-pressure fuel in the vapor separator to its boiling point and cause the fuel to vaporize. When this occurs, the high pressure pump becomes starved of fuel. This causes rough engine operation and stalling of the engine. Cooling of the vapor separator or the high pressure return fuel line counteracts, to a degree, the accumulation of heat and the resulting vaporization of the fuel. However, despite the cooling of the vapor separator and/or high pressure pump, heat from the engine and other surrounding components is absorbed by the fuel rail.
Water cooled fuel rails are used to, at least partially, remove the heat absorbed by the fuel rail from its ambient environment. Water cooled fuel rails, such as those used in marine engines, typically draw cooling water from the body of water in which the boat or engine is operating. The cooling water is circulated through a coolant conduit associated with the fuel rail. The fuel rail includes a fuel conduit which is adjacent to the coolant conduit and carries the fuel. The two conduits are typically separated by and share an inner wall. Such fuel rails are typically machined aluminum extrusions that are anodized for protection against the corrosive effects of the cooling water.
However, the extruded design dictates the walls of the fuel rail also be aluminum, which promotes unwanted heat transfer between the fuel rail and its ambient environment. Aluminum conducts heat 500 times faster than certain composite materials and about 10 times faster than stainless steel. The parallel configuration of the conduits in the extruded design results in the outer surface of the coolant conduit being exposed to the ambient environment which is heated by the engine and surrounding components. Thus, heat from the engine and other components is readily absorbed by the thermally conductive aluminum fuel rail, thereby creating inefficiency in the cooling of the fuel rail.
To form the rail, the extruded blank requires extensive machining during which a substantial portion of the blank material is removed thereby resulting in wasted raw material. Features, such as injector ports, mounting holes, and interfaces for the fuel inlet, fuel outlet, coolant inlet and coolant outlet, also must be machined into the blank. Other features, such as bolt flanges, are formed of a thick section of aluminum that extends the entire length of the fuel rail which adds undesirable mass and thermal inefficiencies to the rail. Such added mass can be machined away, but that process adds to the time and expense involved in manufacturing extruded fuel rails and the amount of raw material which is wasted.
The parallel conduits within the fuel rail share a common wall which separates the conduits, and through which most of the heat transfer or cooling occurs. As water is circulated through the coolant conduit, it contacts and removes heat from the wall shared by and separating the conduits. Thus, the surface area available for the transfer of heat away from the rail is limited to the surface area of that one common wall. Furthermore, the extrusion process dictates a minimum wall thickness of about 3 mm. Thus, the common wall separating the coolant conduit from the fuel conduit can be no thinner than about 3 mm. Heat transfer is inversely proportional to thickness. Thus, the relatively thick common cooling wall and the decreased thermal conductivity thereof makes cooling the rail even more inefficient.
The process of extruding rails typically requires large machine complexes in order to obtain efficiency. Thus, a large investment in equipment is required, and such equipment must be dedicated to a particular fuel rail design. The machines make the implementation of a change in the design of the fuel rail or the production of a new fuel rail time consuming, expensive and difficult, and typically requires retooling. Once a change is implemented, it may be impossible to produce a previous fuel rail model without reversing those difficult and expensive changes. Furthermore, the aluminum blank must typically be sent to an outside contractor for anodization, thereby adding further time and expense to the manufacture of such fuel rails.
The extrusion process also limits the flow area, or cross-sectional area, of the fuel conduit. The mass of an extruded fuel rail is directly proportional to its cross- sectional area. The larger the cross-sectional area, the greater the mass of an extruded rail. An increase in mass adversely affects the thermal characteristics of a fuel rail. Thus, the cross-sectional area of an extruded rail is limited by the adverse thermal consequences a particular design can incur. A fuel conduit with a larger cross section offers advantages in fluid dynamics. Dynamic pressure pulsations within a fuel conduit result from the operation of the fuel injectors connected to the fuel conduit. The activation of a fuel injector causes a rapid change in the fuel flow rate through the conduit. The rapid change in fuel velocity sends a pressure wave through the fuel conduit. The change in fuel velocity is inversely proportional to the flow area, or cross sectional area, of the fuel conduit. This phenomenon is analogous to the pressure wave in a water pipe when a faucet is suddenly turned on or off. The pressure wave within a fuel conduit adversely affects the operation of the fuel injectors being supplied by the conduit, and can cause rough engine operation, stalling, an increase in undesirable emissions and objectionable noise. Thus, fuel dynamics are adversely affected by the limit placed on the cross-sectional area of an extruded fuel conduit.
Therefore, what is needed in the art is a liquid cooled fuel rail assembly which utilizes thermally dissimilar materials that do not readily absorb heat from the ambient environment into the fuel rail assembly, and yet provides efficient heat transfer from the fuel conduit to the coolant conduit. These materials should permit a reduction in the overall mass of the fuel rail assembly to thereby further promote efficient heat transfer.
Furthermore, what is needed in the art is a liquid cooled fuel rail assembly which maximizes the surface area through which heat is transferred from the fuel conduit to the coolant conduit.
Moreover, what is needed in the art is a liquid cooled fuel rail assembly which allows the use of a thin separating wall between the coolant and fuel conduits, thereby minimizing the thermal resistance between the two conduits.
Yet further, what is needed in the art is a liquid cooled fuel rail assembly which can have an enlarged flow area, or cross sectional area, thereby decreasing the adverse effects of the pressure wave created within the fuel conduit by the operation of the fuel injectors.
Even further, what is needed in the art is a liquid cooled fuel rail assembly which can be produced using more efficient, more flexible and less expensive manufacturing processes.